U.S. patent number 8,957,386 [Application Number 13/564,672] was granted by the patent office on 2015-02-17 for doped cesium barium halide scintillator films.
This patent grant is currently assigned to Radiation Monitoring Devices, Inc.. The grantee listed for this patent is Harish B. Bhandari, Vivek V. Nagarkar. Invention is credited to Harish B. Bhandari, Vivek V. Nagarkar.
United States Patent |
8,957,386 |
Nagarkar , et al. |
February 17, 2015 |
Doped cesium barium halide scintillator films
Abstract
Strontium halide scintillators, calcium halide scintillators,
cerium halide scintillators, cesium barium halide scintillators,
and related devices and methods are provided.
Inventors: |
Nagarkar; Vivek V. (Weston,
MA), Bhandari; Harish B. (Brookline, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nagarkar; Vivek V.
Bhandari; Harish B. |
Weston
Brookline |
MA
MA |
US
US |
|
|
Assignee: |
Radiation Monitoring Devices,
Inc. (Watertown, MA)
|
Family
ID: |
52463609 |
Appl.
No.: |
13/564,672 |
Filed: |
August 1, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
13460705 |
Apr 30, 2012 |
|
|
|
|
61480325 |
Apr 28, 2011 |
|
|
|
|
61514000 |
Aug 1, 2011 |
|
|
|
|
Current U.S.
Class: |
250/362 |
Current CPC
Class: |
C09K
11/7719 (20130101); C23C 14/243 (20130101); G01T
1/202 (20130101); C23C 14/0694 (20130101); G01T
1/2023 (20130101); C09K 11/7733 (20130101); C23C
14/24 (20130101); G21K 2004/06 (20130101) |
Current International
Class: |
G01T
1/20 (20060101) |
Field of
Search: |
;250/362,361R,370.11,366,367 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
US. Appl. No. 12/721,505, filed Mar. 10, 2010, Nagarkar et al.
cited by applicant .
U.S. Appl. No. 13/460,626, filed Apr. 30, 2012, Nagarkar et al.
cited by applicant .
U.S. Appl. No. 13/460,705, filed Apr. 30, 2012, Nagarkar. cited by
applicant .
Babla et al., "A triple-head solid state camera for cardiac single
photon emission tomography," Proc. of SPIE 6319:63190M 1-5 (2006).
cited by applicant .
Bartzakos & Thompson, "A PET detector with depth-of-interaction
determination," Phys. Med. Biol. 3 6(6): 735-748 (1991). cited by
applicant .
Burr et al., "Evaluation of a prototype small-animal PET detector
with depth-of-interaction encoding," IEEE Trans. NucL Sci.
51(4):1791-1798 (2004). cited by applicant .
Derenzo et al., "Initial characterization of a position-sensitive
photodiode/BGO detector for PET," IEEE Trans. NucL Sci. 36(1):1-6
(1989). cited by applicant .
Gramsch, "Measurement of the depth of interaction of an LSO
scintillator using a planar process ADP," IEEE Trans. on NucL Sci.
50 (3):307-312 (2003). cited by applicant .
Huber et al., "An LSO scintillator array for a PET detector module
with depth of interaction measurement," IEEE Trans. Nucl. Sci.
48:684-688 (2001). cited by applicant .
Inadama et al., "A Depth of Interaction Detector for PET with GSO
Crystals doped with Different amount of Ce," IEEE, 1099-1103
(2002). cited by applicant .
Karp & Daube-Witherspoon, "Depth-of-interaction determination
in Nal(T1) and BGO scintillation crystals using a temperature
gradient," Nucl. Instr. Methods Phys. Res. A260:509-517 (1987).
cited by applicant .
Knoll, "Pulse Shape Discrimination," in: Radiation Detection and
Measurement, Third Edition, Glenn F. Knoll, John Wiley & Sons,
NY, p. 646 (2000). cited by applicant .
Knoll, "Specialized Detector Configurations Based on
Scintillation," in: Radiation Detection and Measurement, Second
Edition, John Wiley & Sons, NY, p. 344-345 (1989). cited by
applicant .
Kupinski and Barrett, Small-Animal SPECT Imaging, Springer
Science+Business Media Inc. (2005). cited by applicant .
Ling et al., "Depth of interaction decoding of a continuous crystal
detector module," Phys. Med. Biol. 52:2213-2228 (2007). cited by
applicant .
MacDonald & Dahlbom, "Depth of interaction for PET using
segmented crystals," IEEE Trans. Nucl. Sci. 45(4):2144-2148 (1998).
cited by applicant .
Miyaoka et al., "Design of a depth of interaction (DOI) PET
Detector Module," IEEE Trans. on Nucl. Sci. 45(3):1069-1073 (1998).
cited by applicant .
Moisan et al., "Segmented LSO crystals for depth-of-interaction
encoding in PET," IEEE Trans. Nucl. Sci. 45(6):3030-3035 (1998).
cited by applicant .
Murayama et al., "Design of a depth of interaction detector with a
PS-PMT for PET," IEEE Trans. Nucl. Sci. 47(3):1045-1050 (2000).
cited by applicant .
Nagarakar at al., "Development of microcolumnar Labr3:Ce
scintillator," Proc. of SPIE 7450:745006-1-745006-10 (2009). cited
by applicant .
Nagarakar et al., "Microcolumnar and polycrystalline growth of
LaBr3:Ce scintillator," Nucl. Instr. and Meth. A (2010),
doi:10.1016/j.nima.2010.06.190. cited by applicant .
Saoudi at al., "Investigation of depth-of-interaction by pulse
shape discrimination in multicrystal detectors read out by
avalanche photodiodes," IEEE Trans. Nucl. Sci. 46(3):462-467
(1999). cited by applicant .
Schramm et al., High-resolution SPECT using multi-pinhole
collimation, IEEE Trans. Nucl. Sci. 50(3):774-777 (2003). cited by
applicant .
Seidel et al., "Depth identification accuracy of a three layer
phoswich PET detector module," IEEE Trans. Nucl. Sci. 46(3):485-490
(1999). cited by applicant .
Shah at al., "LcC13:Ce scintillator for y-ray detection," Nucl.
Instr. and Meth. Phys. Res. A 505: 76-81 (2003). cited by applicant
.
Shao et al., "Dual APD array readout of LSO crystals: optimization
of crystal surface treatment," IEEE Trans. Nucl. Sci. 49(3):649-654
(2002). cited by applicant .
Smith et al., "Design of multipinhole collimators for small animal
SPECT," IEEE NSS/MIC Conference Records (2004). cited by applicant
.
Stahle et al., "Fabrication of CdZnTe strip detectors for large
area arrays," SPIE 3115:90-97 (1997). cited by applicant .
Streun et al., "Pulse shape discrimination of LSP and LuYAP
scintillators for depth of intereaction detection in PET," IEEE
Trans. Nucl. Sci. 50(3):344-347 (2003). cited by applicant .
Tornai et al., Comparison of compact gamma cameras with 1.3- and 2
0-mm quantized IEEE Trans. Nucl. Sci. 52(5):1251-1256 (2005). cited
by applicant .
Yamamoto & Ishibashi, "A GSO depth of interaction detector for
PET," IEEE Trans. NucNucl.Sci. 45(3): 1078-1082 (1998). cited by
applicant .
Yamashita et al., "High resolution block detectors for PET," IEEE
Trans. Nucl. Sci. 37(2):589-593 (1990). cited by applicant.
|
Primary Examiner: Porta; David
Assistant Examiner: Boosalis; Faye
Attorney, Agent or Firm: Wilson Sonsini Goodrich &
Rosati
Government Interests
STATEMENT OF GOVERNMENT RIGHTS
This work was supported by the United States Army Medical Research
Acquisition Activity under Grant No. W81XWH12C0057. The U.S.
Government may have certain rights in this invention.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
The present application is a continuation-in-part of U.S. patent
application Ser. No. 13/460,705, filed Apr. 30, 2012, which claims
priority to U.S. Provisional Application No. 61/480,325, filed Apr.
28, 2011, and claims priority to U.S. Provisional Application No.
61/514,000, filed Aug. 1, 2011, each of which are incorporated by
reference in their entirety.
Claims
What is claimed is:
1. A scintillator comprising, a ternary scintillator compound
comprising a dopant and cesium barium halide, wherein the
scintillator is in the form of a microcolumnar scintillator film or
a noncolumnar polycrystalline film, and wherein the scintillator
has the formula CsBa.sub.2I.sub.5:Eu or
CsBa.sub.2I.sub.5:Eu+Tl.
2. The scintillator of claim 1, wherein the scintillator has a
thickness of less than about 20 microns.
3. The scintillator of claim 1, wherein the scintillator has a
thickness greater than about 1 cm.
4. The scintillator of claim 1, wherein the scintillator is in the
form of an amorphous microcolumnar scintillator film or a
crystalline microcolumnar scintillator film.
5. The scintillator of claim 1, wherein the scintillator comprises
less than 0.05% afterglow at 2 ms following X-ray excitation.
6. The scintillator of claim 1, wherein the scintillator further
comprises a coating layer comprising doped or undoped cesium
iodide.
7. A radiation detection device comprising a scintillator of claim
1 and a photodetector assembly optically coupled to the
scintillator.
8. The radiation detection device of claim 7, further comprising
electronics coupled to the photodetector assembly so as to output
image data in response to radiation detected by the
scintillator.
9. A method of performing radiation detection, comprising:
providing a detector device comprising a scintillator of claim 1
and a photodetector assembly optically coupled to the scintillator;
and positioning a radiation source within a field of view of the
scintillator so as to detect emissions from the radiation
source.
10. The scintillator of claim 1, wherein the scintillator has a
thickness of less than about 500 microns.
11. The scintillator of claim 1, wherein the scintillator has a
thickness of less than about 1 cm.
12. The scintillator of claim 1, wherein the scintillator has a
thickness greater than about 3 cm.
13. The scintillator of claim 1, wherein the scintillator has a
thickness greater than about 5 cm.
14. A hot wall evaporation method of making a doped cesium barium
halide scintillator, the method comprising: providing an
evaporation apparatus comprising an evaporation chamber having a
first end portion with a substrate positioned in a holder, a second
end portion with at least one source boat, and one or more chamber
walls at least partially disposed between the first and second end
portions; positioning a dopant and a source at the second end
portion so as to allow evaporation of the dopant and the source
into the evaporation apparatus; and depositing a scintillator film
comprising the dopant and the source on a surface of the positioned
substrate, the scintillator film comprising doped cesium barium
halide having the formula CsBa.sub.2I.sub.5:Eu or
CsBa.sub.2I.sub.5:Eu+Tl.
15. The method of claim 14, wherein the film is deposited by a
process comprising applying heat to the evaporation chamber so as
to vaporize the dopant and the source for film deposition while
maintaining a temperature relationship of
T.sub.wall>T.sub.source>T.sub.substrate for at least a
portion of the deposition process.
16. The method of claim 14, wherein T.sub.source is between about
300.degree. C. and about 750.degree. C.
17. The scintillator of claim 14, wherein the scintillator
comprises less than 0.05% afterglow at 2 ms following X-ray
excitation.
18. The method of claim 14, wherein the dopant and the source are
included in melt-growth polycrystals positioned in one source boat
at the second end portion.
19. The method of claim 18, wherein the melt-growth crystal is
synthesized from CsI, BaI.sub.2 and EuI.sub.2.
20. A scintillator produced by the method of claim 14.
21. A deposition method of making a doped cesium barium halide
scintillator, the method comprising: providing a deposition
apparatus comprising a vacuum chamber having a substrate at a first
end and at least one source boat at a second end of the chamber;
positioning a source material and a dopant material in the at least
one source boat; and depositing a scintillator film comprising
doped cesium barium halide on the substrate, wherein the doped
cesium barium halide has the formula CsBa.sub.2I.sub.5:Eu or
CsBa.sub.2I.sub.5:Eu+Tl.
22. The method of claim 21, wherein the dopant material and the
source material are included in melt growth polycrystals positioned
in one source boat at the second end portion.
23. The method of claim 21, wherein the apparatus comprises a first
source boat comprising cesium iodide, a second source boat
comprising barium iodide, and a third source boat comprising
europium iodide.
24. The method of claim 23, wherein the apparatus further comprises
a fourth source boat comprising thallium iodide.
25. The method of claim 21, wherein the scintillator has a
thickness of less than about 20 microns.
26. The method of claim 21, wherein the scintillator has a
thickness greater than about 1 cm.
27. The method of claim 21, wherein the scintillator is in the form
of an amorphous microcolumnar scintillator film or a crystalline
microcolumnar scintillator film.
28. A scintillator produced by the method of claim 21.
29. The method of claim 21, wherein the scintillator has a
thickness of less than about 500 microns.
30. The method of claim 21, wherein the scintillator has a
thickness of less than about 1 cm.
31. The method of claim 21, wherein the scintillator has a
thickness greater than about 3 cm.
32. The method of claim 21, wherein the scintillator has a
thickness greater than about 5 cm.
Description
BACKGROUND OF THE INVENTION
The present invention relates to scintillator fabrication methods
and scintillators. More specifically, the present invention
provides a variety of scintillators, including strontium halide
scintillators, calcium halide scintillators, cerium halide
scintillators and cesium barium halide scintillators. Related
devices and methods of using the scintillators described herein are
also provided.
Scintillation spectrometers are widely used in detection and
spectroscopy of energetic photons (e.g., X-rays and g-rays). Such
detectors are commonly used, for example, in nuclear and particle
physics research, medical imaging, diffraction, non-destructive
testing, nuclear treaty verification and safeguards, nuclear
non-proliferation monitoring, and geological exploration.
Important requirements for the scintillation materials used in
these applications include high light output, transparency to the
light it produces, high stopping efficiency, fast response, good
proportionality, low cost and availability in large volume. These
requirements are often not met by many of the commercially
available scintillators. While general classes of chemical
compositions may be identified as potentially having some
attractive scintillation characteristic(s), specific
compositions/formulations and structures having both scintillation
characteristics and physical properties necessary for actual use in
scintillation spectrometers and various practical applications, as
well as capability of imaging at a high resolution, have proven
difficult to predict or produce. Specific scintillation properties
are not necessarily predictable from chemical composition alone,
and preparing effective scintillators from even candidate materials
often proves difficult. For example, while the composition of
sodium chloride had been known for many years, the invention by
Hofstadter of a high light-yield and conversion efficiency
scintillator from sodium iodide doped with thallium launched the
era of modern radiation spectrometry. More than half a century
later, thallium doped sodium iodide, in fact, still remains one of
the most widely used scintillator materials. Since the invention of
NaI(Tl) scintillators in the 1940's, for half a century radiation
detection applications have depended to a significant extent on
this material. As the methodology of scintillator development
evolved, new materials have been added, and yet, specific
applications, particularly those requiring high resolution imaging
and large volumes, are still hampered by the lack of scintillators
suitable for particular applications.
As a result, there is continued interest in the search for new
scintillator formulations and physical structures with both the
enhanced performance and the physical characteristics needed for
use in various applications. Today, the development of new
scintillators continues to be as much an art as a science, since
the composition of a given material does not necessarily determine
its performance and structural properties as a scintillator, which
are strongly influenced by the history (e.g., fabrication process)
of the material as it is formed. While it is may be possible to
reject a potential scintillator for a specific application based
solely on composition, it is not possible to predict whether a
material with promising composition will produce a scintillator
with the desired properties.
Thus, a need exists for scintillators that have imaging capability
with improved properties, such as spatial and/or temporal
resolution, and methods of making the scintillators.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to scintillator fabrication methods
and scintillators. The scintillators of the invention are useful in
a variety of applications including, for example, spectroscopy
detection of energetic photons (X-rays and gamma-rays) and imaging
applications (e.g., X-ray imaging, PET (positron emission
tomography), etc.).
In one aspect, the present invention provides a scintillator
comprising, a dopant and a scintillator material selected from the
group consisting of strontium halide, calcium halide, and cesium
barium halide, wherein the scintillator is in the form of a
microcolumnar scintillator film or a noncolumnar polycrystalline
film. In one embodiment, the present invention provides a
scintillator comprising a dopant and cesium barium halide. The
scintillator films of the present invention can include
SrI.sub.2:Eu, CaI.sub.2:Eu, CeBr.sub.3, or CsBa.sub.2I.sub.5:Eu. In
some embodiments, the scintillator films can include CaI or
CeBr.sub.3.
In another aspect, the present invention provides a radiation
detection device including at least one of the scintillators
provided herein (e.g., SrI.sub.2:Eu, CaI.sub.2:Eu, CaI, CeBr.sub.3,
or CsBa.sub.2I.sub.5:Eu); and a photodetector assembly optically
coupled to the scintillator.
In yet another aspect, the present invention provides a method of
performing radiation detection. The method can include providing a
detector device including a scintillator provided herein (e.g.,
SrI.sub.2:Eu, CaI.sub.2:Eu, CaI, CeBr.sub.3, or
CsBa.sub.2I.sub.5:Eu); and a photodetector assembly optically
coupled to the scintillator; and positioning a radiation source
within a field of view of the scintillator so as to detect
emissions from the radiation source.
In yet another aspect, the present invention provides methods of
making a scintillator. The methods can include providing an
evaporation apparatus comprising an evaporation chamber having a
first end portion with a substrate positioned in a holder, and a
second end portion with a first source boat separate from a second
source boat, and one or more chamber walls at least partially
disposed between the first and second end portions; positioning a
dopant salt in the first source boat and a source salt in the
second source boat; and depositing a scintillator film comprising a
source and a dopant on a surface of the positioned substrate,
wherein the source salt is selected from the group consisting of
strontium halide, calcium halide, and cesium barium halide. In some
embodiments, the hot wall evaporation techniques described herein
can also be used to make SrI.sub.2:Eu, CaI.sub.2:Eu, or
CsBa.sub.2I.sub.5:Eu scintillator films. In some embodiments, the
hot wall evaporation techniques described herein can also be used
to make CeBr.sub.3 or CaI scintillator films.
In yet another aspect, the present invention includes a deposition
method of making a doped cesium barium halide scintillator. The
method can include providing a deposition apparatus comprising a
vacuum chamber having a substrate at a first end and a scintillator
source boat at a second end of the chamber; positioning a source
material and a dopant material in the scintillator source boat; and
depositing a scintillator film comprising doped cesium barium
halide on the substrate.
For a fuller understanding of the nature and advantages of the
present invention, reference should be had to the ensuing detailed
description taken in conjunction with the accompanying
drawings/figures. The drawings/figures represent embodiments of the
present invention by way of illustration. The invention is capable
of modification in various respects without departing from the
invention. Accordingly, the drawings/figures and description of
these embodiments are illustrative in nature, and not
restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example scintillator film, according to one
embodiment of the present invention.
FIG. 2 shows a hot wall evaporation method, according to an
embodiment of the present invention.
FIG. 3 shows a hot wall evaporation apparatus, according to an
embodiment of the present invention.
FIG. 4 shows a hot wall evaporation apparatus, according to an
embodiment of the present invention.
FIG. 5 shows an example detection device, according to an
embodiment of the present invention.
FIG. 6 shows a microcolumnar structure of a SrI.sub.2:Eu
scintillator film, according to an embodiment of the present
invention.
FIGS. 7A and 7B provide spectra of a cerium bromide scintillator
film, according to an embodiment of the present invention.
FIGS. 8A and 8B shows a fabricated CsBa.sub.2I:Eu scintillator film
and comparison spectra for the scintillator, respectively,
according to an embodiment of the present invention.
FIG. 9 shows melt-grown crystals synthesized at using a vertical
Bridgman furnace, according to an embodiment of the present
invention.
FIG. 10 depicts a comparison between light yield of
Ba.sub.2CsI.sub.5:Eu polycrystals and Ba.sub.2CsI.sub.5:Eu
scintillator films at varied Eu.sup.2' concentrations, according to
an embodiment of the present invention.
FIG. 11 shows an example thermal evaporation apparatus, according
to an embodiment of the present invention.
FIG. 12 provides a scanning electron microscope image of a cesium
barium halide scintillator, according to an embodiment of the
present invention.
FIG. 13 illustrates a spectral comparison between a
Ba.sub.2CsI.sub.5:Eu scintillator film and an annealed
Ba.sub.2CsI.sub.5:Eu scintillator film, according to an embodiment
of the present invention.
FIG. 14 shows an emission spectrum of a Ba.sub.2CsI.sub.5:Eu+Tl,
dual-doped scintillator film, according to an embodiment of the
present invention.
FIG. 15 depicts a line phantom image acquired using a
Ba.sub.2CsI.sub.5:Eu scintillator film, according to an embodiment
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to scintillator fabrication methods
and scintillators. More specifically, the present invention
provides a variety of scintillators, including strontium halide
scintillators, calcium halide scintillators, cerium halide
scintillators and cesium barium halide scintillators. Related
devices and methods of using the scintillators described herein are
also provided.
The methods and structures described herein can be used to make a
variety of scintillator compositions. For example, the present
invention provides, but is not limited to, strontium halide
scintillators, calcium halide scintillators, cerium halide
scintillators, or cesium barium halide scintillators. Halides can
include fluoride, chloride, bromide and/or iodide. The
scintillators can include strontium iodide-based (e.g., SrI.sub.2)
scintillator compositions. In some embodiments, the scintillators
include cerium bromide (e.g., CeBr.sub.3) scintillator
compositions. Cesium barium iodide-based (e.g., CsBa.sub.2I.sub.5)
scintillator compositions are also provided. Calcium iodide
scintillators (e.g., CaI.sub.2, CaI.sub.2:Eu, and CaI.sub.2:Tl) are
further provided herein. Other halide scintillators, include, but
are not limited to, doped SrBr.sub.2, CaBr.sub.3, and
CeCl.sub.3.
The scintillator compositions of the present invention can
optionally include a "dopant." In certain embodiments, the
scintillators described herein will typically include a dopant.
Dopants can affect certain properties, such as physical properties
(e.g., brittleness, etc.) as well as certain scintillation
properties (e.g., afterglow, etc.) of the scintillator composition.
The dopant can include, for example, europium (Eu), praseodymium
(Pr), cerium (Ce), thallium (Tl), terbium (Tb), ytterbium (Yb), and
mixtures of any of the dopants. The amount of dopant present will
depend on various factors, such as the application for which the
scintillator composition is being used; the desired scintillation
properties (e.g., emission properties, timing resolution, etc.);
and the type of detection device into which the scintillator is
being incorporated. For example, the dopant is typically employed
at a level in the range of about 0.1% to about 20%, by molar
weight. In some embodiments, the amount of dopant is in the range
of about 0.1% to about 100%, or about 0.1% to about 5.0%, or about
5.0% to about 20%, by molar weight.
In one embodiment, the scintillators can include SrI.sub.2:Eu
scintillator compositions. In another embodiment, the scintillators
can include CsBa.sub.2I.sub.5:Eu scintillator compositions. As will
be appreciated by one of ordinary skill in the art, cesium barium
iodide is also termed barium cesium iodide (or europium-doped
barium cesium iodide, or Ba.sub.2CsI.sub.5:Eu). Doped cesium barium
halide scintillator films of the present invention can also include
dual-doped scintillator films, e.g., Ba.sub.2CsI.sub.5:Eu+Tl. In
some embodiments, the scintillators can include CaI.sub.2,
CaI.sub.2:Eu, or CaI.sub.2:Tl scintillator compositions.
The scintillators described herein (e.g., SrI.sub.2:Eu,
CaI.sub.2:Eu, CeBr.sub.3, or CsBa.sub.2I.sub.5:Eu scintillators)
can be produced in a variety of different forms. As shown in FIG.
1, the scintillators can be a film 10 deposited on a substrate 12.
For example, the scintillators described herein can be
polycrystalline or crystalline. In certain embodiments, the
scintillators described herein can be microcolumnar scintillators.
Column widths of the microcolumnar scintillators can be, e.g.,
greater than about 5 microns in width. In some embodiments, the
columns can have widths between 5 microns and 100 microns, between
100 microns and 1 millimeter, and higher. An upper limit for the
microcolumns can be a width that is the same as the thickness of
the film. In some embodiments, the scintillators described herein
are in the form of a polycrystalline film. The scintillators can be
a microcolumnar scintillator film. In some embodiments,
scintillators can be produced as transparent or translucent
scintillators that include a crystalline or polycrystalline
layer(s).
The present invention further includes doped cesium barium halide
scintillators (e.g., CsBa.sub.2I:Eu) in various forms, including an
amorphous microcolumnar structure form or a crystalline
microcolumnar structure form. The doped ternary scintillator,
CsBa.sub.2I:Eu, can exist as a mono-phase that is not a solid
solution of Cs, Ba, I, but rather a compound with chemical bonding
amongst Cs, Ba, and I, as well as Eu. The ternary compound
structure can be unstable under certain conditions. Accordingly, to
produce an optimally functioning scintillator, a scintillator in
the mono-phase can be produced in which, e.g., Cs, Ba, I and Eu can
be kinetically trapped in an arrangement that generates excellent
scintillation properties. For example, Ba.sub.2CsI.sub.5 is formed
in-part from the reaction of BaI.sub.2 and CsI binary compounds.
However, while not being limited to any particular theory, when
energy is applied over a long period of time on such ternary
compounds, the thermodynamically stable product will result, which
are BaI.sub.2 and CsI, not Ba.sub.2CsI.sub.5. Binary compounds can
exhibit higher lattice stability owing, e.g., to their simplicity
in bond formation. Hence, ternary compounds can be less stable and
difficult to form if the thermodynamics prefer more stable binary
compounds. For Ba.sub.2CsI.sub.5, the melting points of CsI and
BaI.sub.2 are 630.degree. C. and 740.degree. C. However, when a
mono-phase of Ba.sub.2CsI.sub.5 is synthesized from the reaction of
the two binary phases, the melting point of the ternary phase is
.about.600.degree. C., which is lower than its constituent binary
phases. With a lower melting point than CsI, Ba.sub.2CsI.sub.5
would be expected to have a higher vapor pressure than CsI.
However, via indirect experimentation it was discovered that the
vapor pressure for Ba.sub.2CsI.sub.5 was lower, not higher, than
CsI. Typically, to improve vapor pressure of a material, the
temperature can be increased. However, increased temperature for
Ba.sub.2CsI.sub.5, e.g., beyond its melting point can cause phase
separation into BaI.sub.2 and CsI. Owing in part to this discovery,
the methods (e.g., hot wall evaporation, thermal evaporation or
physical vapor deposition) of making doped cesium barium halide
scintillator films described herein account for this lower vapor
pressure and allow for fabrication of the doped cesium barium
halide scintillator films. The methods can also include other ways
to address the properties of doped cesium barium halide
scintillator films. For example, a finely powdered charge of
Ba.sub.2CsI.sub.5:Eu can be sublimated from a customized boat that
facilitates a large surface area and will provide enough
vaporization compared to single planar surface of liquid-melt. In
addition, as described further herein, the present invention
further includes making doped cesium barium halide polycrystals
(e.g., Ba.sub.2CsI.sub.5:Eu polycrystals) that have a monophase
prior to vaporization, which can be used to generate monophase
films (e.g., Ba.sub.2CsI.sub.5:Eu films).
The scintillators described herein can be produced in a variety of
thicknesses and spatial areas. Thicknesses of the films can be
designed for certain imaging applications. For example, thin films
(e.g., less than 20 .mu.m) can be used to detect alpha particles,
while limiting interference from detection of gamma rays. Thickness
of the films can also be tailored to detect, for example, gamma
rays, while also allowing for sufficient light transmission.
Spatial resolution for, e.g., microcolumnar scintillators, can also
be tailored by thickness of the films. In some embodiments,
scintillators can be produced as a thin film, e.g., films having a
thickness ranging from about 10 .mu.m to about 1 cm. In certain
embodiments, the scintillators can be thick films having a
thickness of 1 cm or greater, and can be referred to as "slabs." In
some embodiments, scintillators can have thickness of less than
about 20 microns, less than about 500 microns, or less than about 1
cm. In certain embodiments, the scintillators can have a thickness
greater than about 1 cm, greater than about 3 cm, or greater than
about 5 cm. The scintillators described herein can be freestanding
films, e.g., where the deposited film can be removed from the
substrate after deposition. The scintillators can be deposited over
small to wide areas on the order, for example, of mm.sup.2 or
cm.sup.2 (e.g., up to 50.times.50 cm.sup.2).
Scintillator compositions and assemblies of the present invention
can further include one or more reflective coatings, e.g., formed
on a substrate surface or otherwise coupled with a scintillator. In
one embodiment, a reflective coating can be formed on a substrate
surface prior to deposition of the scintillator, such as in the
case of opaque substrates. One embodiment includes the use of
substrates such as alumina that are themselves white in color and
act as an excellent reflector. Another embodiment includes the use
of substrates that are themselves photodetectors (e.g., SSPMs,
amorphous silicon arrays, CCDs, and CMOS devices). For
graphite-like substrates, reflective metal coatings can be formed.
In addition to being highly reflective, such coatings may be
required to withstand high process temperatures, maintain adhesion
to the substrate during and after deposition, and/or be chemically
inert with the scintillator or suitable for coating with a
chemically inert material, such as an organic polymer or resin
(e.g., Parylene C). For transparent substrates a reflective coating
can be applied atop the scintillator film directly or after
deposition of one or more other coatings, such as a Parylene
coating. As noted above, various coating technologies can be
utilized for forming coatings with the required optical and/or
protective properties.
In some embodiments, a protective coating can be deposited or
placed over the scintillator film to protect it, e.g., from air
and/or moisture. For example, a Ba.sub.2CsI.sub.5:Eu scintillator
film is hygroscopic and can degenerate when exposed to moisture. In
one embodiment, a parylene film can be deposited on the
scintillator. In an alternative embodiment, another less
hygroscopic scintillator can be coated on the Ba.sub.2CsI.sub.5:Eu
scintillator film. For instance, a doped or undoped cesium halide
scintillator film (e.g., that is tens of microns in thickness) can
be deposited near the end of the deposition cycle of the
Ba.sub.2CsI.sub.5:Eu scintillator film. A source boat including
cesium halide can, e.g., be evaporated during the deposition cycle
to deposit the cesium halide over the Ba.sub.2CsI.sub.5:Eu
scintillator film. In some embodiments, a parylene film can also be
added on top of the cesium halide film. Experimental evidence has
further shown, e.g., that a top layer of CsI of a few microns is
transparent to the light emitted in the scintillator and also
continues to grow along the same columnar geometry as the
scintillator.
The present invention includes methods and structures for
fabricating the scintillators described herein. In some
embodiments, the scintillators described herein can be grown using
a vapor deposition technique, co-evaporation in a hot wall
evaporation (HWE) apparatus. In certain embodiments, the hot wall
evaporation techniques can include hot wall epitaxial growth of the
scintillators described herein. Hot wall evaporation techniques, as
described herein, include a vacuum deposition technique where
scintillator film is efficiently deposited on a surface of a
substrate. In the simplest form the HWE apparatus includes a
chamber or cylinder positioned in a vacuum, heated, with an
evaporation source "boat" or reservoir at one end (typically the
bottom in an upright positioned chamber) and a temperature
controlled substrate at the other (typically the top in an upright
positioned chamber). In certain embodiments, the disclosed methods
make use of salts and vapor deposits them, e.g., simultaneously, on
a suitable substrate using two independent sources. Under different
deposition conditions, the material grows in the desired form. In
one embodiment, two source boats are used to accomplish
co-evaporation of a salt and a dopant salt for deposition of a
scintillator film on a substrate surface. In some embodiments, the
HWE techniques can include three or more source boats. Each boat
can contain a particular source material (a.k.a. charge) of
interest to produce the scintillators described herein. The source
boats can be positioned separate (e.g., laterally spaced) from each
other.
FIG. 2 provides an example method 20 for performing hot wall
evaporation to make a doped scintillator film, such as a doped
cesium halide scintillator. In general, the method can include
providing a hot wall evaporation apparatus having a substrate, one
or more source boats and one or more chamber walls disposed between
the substrate and the one or more source boats 22. Material in the
one or more source boats can be deposited onto the substrate 24,
and a scintillator material (e.g., a strontium halide, calcium
halide, cerium halide, or cesium barium halide scintillator film)
can be formed on the substrate 26.
FIG. 3 shows an example hot wall evaporation apparatus 30 for use
in the present invention, e.g., for making a doped cesium barium
halide scintillator film. A first precursor boat 31 (e.g., CsI and
BaI.sub.2 starting material) and a first precursor boat 32 (e.g.,
EuI.sub.2 dopant material) are co-located at one end of a hot wall
column 33 (e.g., a metallic cylinder). Heater electrodes 34 are
located next to the precursor boats, and resistive heaters 35 are
provided around the hot wall column to heat to a temperature,
T.sub.wall. A substrate 36 is positioned at the other end of the
hot wall column opposing the precursor boats 31, 32. The substrate
36 is contacted to a cooled substrate holder 37 that can be cooled,
e.g., by a liquid cooling system 38. The components of the
apparatus 30 are provided in a vacuum chamber 39. In one
embodiment, temperature ranges for making SrI.sub.2:Eu can be
T.sub.wall from about 750.degree. C. to about 800.degree. C.,
T.sub.source from about 500.degree. C. to about 700.degree. C., and
T.sub.substrate can be from about 300.degree. C. to about
450.degree. C.
FIG. 4 shows another example hot wall evaporation apparatus 40 for
use in the present invention. In one form the HWE apparatus
includes a chamber or cylinder (e.g., a quartz crucible) 41
positioned in a vacuum, heated, with an evaporation source "boat"
or reservoir holding a source material 42 (e.g.,
Ba.sub.2CsI.sub.5:Eu polycrystals) at one end (typically the bottom
in an upright positioned chamber) and a temperature controlled
substrate 43 at the other (typically the top in an upright
positioned chamber). In an example of making SrI.sub.2:Eu, the
temperature of the substrate 43 can be controlled with a
thermocouple 44, such that, e.g., the substrate has a temperature
between about 300.degree. C. and about 450.degree. C. The chamber
or cylinder can be heated by chamber walls including a heater 45
that can include a thermocouple 46 configured to heat the chamber
or cylinder at temperatures, e.g., between about 750.degree. C. to
about 800.degree. C. In general, source material temperatures are
at least the melting points of the materials used; e.g., 5% to 10%
above the source material's .degree. C. melting point.
For HWE, the heated cylinder wall serves to enclose, deflect and
effectively direct the vapor from the source to the substrate where
molecules are deposited with a shallow impinging angle. With the
substrate being the coolest part in the system (e.g., compared to
the cylinder wall and source material), molecules adhere solely or
primarily to the substrate and do not substantially accumulate on
the hot walls, making efficient use of the source material. To
ensure thermodynamic equilibrium the relationship between the
substrate temperature and that of the source and the heated wall
should be: T.sub.wall>T.sub.source>T.sub.substrate. After
deposition, the deposited scintillator material can be annealed in
a variety of atmospheric conditions Annealing can be used to
enhance the performance of the scintillators described herein.
An advantage of HWE is that it preserves the composition of the
grown film with reference to the evaporants even though they have
relatively large differences in vapor pressures and sticking
coefficients. This is due to the fact that HWE takes place under
conditions of thermodynamic equilibrium, which allows the high
vapor pressures of various compounds to be maintained. As a result,
the dissociation of various constituents does not present a problem
for film growth. As a matter of fact, the interaction of components
with each other on the substrate surface can lead, under favorable
growth conditions, to the formation and growth of the
well-structured films.
Deposition of microcolumnar films involves methods where the
evaporated material be incident on the substrate at a grazing
angle. HWE creates this condition through minimized mean free path
for the vaporized molecules due to the large density of evaporated
material, and through efficient reflection of molecules from walls,
which are maintained at the highest temperature in the setup. Thus,
the thermodynamic equilibrium and atmosphere conducive to growth
created by the HWE process allows deposition of stoichiometrically
balanced films with well-separated columnar morphology.
Another aspect of HWE is its high (close to 100%) deposition
efficiency, as the substrate is the coldest part of the evaporation
environment. As a result, vapors that impinge on HWE system parts,
including the hot walls, are deflected and mostly condense only on
the relatively cool substrate. Consequently, material loss is at a
minimum, enhancing the deposition efficiency to 95% or more (and
greatly simplifying apparatus cleaning and maintenance).
A feature of HWE for thick film deposition is that the growth rate
is an order of magnitude higher than that of conventional physical
vapor deposition (PVD) systems. For a single evaporant material
system the deposition rate is proportional to the impingement rate
(O) of atoms on the substrate at constant temperature, and is
governed by the equation (1): O=n(kT/2.PI.m)1/2 (1) where n is the
number of evaporant molecules per unit volume, k is the Boltzmann
constant, T is the source temperature, and m is the mass of the
molecule. For hot wall evaporation, a two-evaporant system, this
equation still holds, since the vapor phases of the constituent
compounds are in equilibrium with the source materials. Therefore,
the two evaporant deposition process is basically very similar to
that for a single evaporant material. As the source temperature T
is very high and the substrate is the coldest part in the
evaporator, the impingement rate of molecules and, hence, the film
growth rate is an order of magnitude higher than with conventional
systems. The growth rate is related to the impingement rate by the
following equation (2): Growth rate={O*Average thickness of a
monolayer}/{Surface density of the scintillator composition} (2)
The process of material growth can consist of a series of events
that begin with the physical adsorption of a fraction of the
incident molecules on the substrate or by forming a stable nucleus
by interaction with the other adsorbed molecules. This process of
nucleation and growth is typical for the formation of a film of one
material on a substrate of a different material. In HWE, no
nucleation takes place, but growth occurs by direct adsorption of
the molecules on low energy sites, such as kinks on an atomic ledge
on the substrate. Under these conditions, even when the growth rate
is very high, a monolayer-by-monolayer deposition is obtained
resulting in excellent stoichiometry of the films.
In some embodiments, there can be additional modifications to the
methods and apparatus for making strontium halide scintillators
(e.g., SrI.sub.2:Eu). For example, SrI.sub.2:Eu is a highly
corrosive material, so a cold trap is used to protect vacuum pumps.
Also, chamber cleaning after deposition is important. Substrate
cooling is necessary, since wall temperatures are high. A planetary
system can be used which rotates at least one substrate around a
system axis as well as around an axis that is normal to the surface
of the substrate. A rate of about 1 to 2 rotations per second would
be used. Deposition parameters (e.g., temperatures) are adjusted
accordingly to achieve a high growth rate of about 10 to about 25
microns per minute, with good thickness uniformity (say, 5% to
10%). For example, gamma detectors use thick sensors for the sake
of detection efficiency, so high growth rates are desirable.
In certain embodiments, temperature control for making strontium
halide can occurs in three stages: (1) Ramp-up: From ambient room
temperature to a range of about 500.degree. C. to about 700.degree.
C., about 3 hours, at a slow rate in order to allow and achieve the
necessary dehydration of source materials before beginning
deposition. Otherwise, moisture in the source materials can
vaporize rapidly, causing undesirable sputtering of source
materials; (2) Deposition: Temperatures are held at the desired
level; e.g., within the range of about 500.degree. C. to about
700.degree. C. Deposition proceeds for a period of time that
depends upon the desired scintillator thickness; (3) Ramp-down: The
temperature is slowly decreased to ambient room temperature, from
about 5 to 10 hours, in order reduce stress on the deposited film.
Vacuum is achieved and held between about 10^-4 to about 10^-7
Torr. Source materials are heated to temperatures that are at least
(e.g., by 5% to 10%) their melting points; e.g., for SrI.sub.2:Eu,
the SrI.sub.2 source would be heated to about 500.degree. C., and
the EuI.sub.2 source would be heated to about 875.degree. C. The
evaporation rates for SrI.sub.2 and EuI.sub.2 to produce
SrI.sub.2:Eu with 2% to 5% Eu doping, for example, can be about 25
microns/minute for SrI.sub.2 and about 0.5 to about 1.25 microns
per minute for EuI.sub.2.
Similar processes can be used to make calcium halide scintillators.
Material vaporization temperatures for the calcium halide source
and dopant materials can be slightly above (e.g., by 5% to 10%)
respective melting points. For CaI.sub.2:Eu and CaI.sub.2:Tl, the
melting point for CaI.sub.2 is about 783.degree. C., EuI.sub.2
about 875.degree. C., and TlI about 300.degree. C. For
Ba.sub.2CsI.sub.5:Eu, the process is the same, except a
vaporization temperature between about 500.degree. C. and about
600.degree. C. can be used for the source material, polycrystalline
Ba.sub.2CsI.sub.5:Eu, which can be prefabricated by melt growth
from an admixture of BaI.sub.2, CsI and EuI.sub.2.
The present invention further includes other deposition methods for
making doped cesium barium halide scintillator films. For example,
a variety of physical vapor deposition apparatus and methods (e.g.,
sputtering or electron beam vaporization) can be used. Thermal
deposition techniques can also be used. In one embodiment, the
methods can include providing a deposition apparatus comprising a
vacuum chamber having a substrate at a first end and scintillator
source boat at a second end of the chamber; positioning a cesium
barium halide source material and a dopant material at the second
end; and depositing a scintillator film comprising doped cesium
barium halide on the substrate. The source material and dopant
material can be provided in a variety of ways. For example, a
single crystal and/or polycrystals of doped cesium barium halide,
e.g., Ba.sub.2CsI.sub.5:Eu, can be synthesized using vertical
Bridgeman growth processes. The crystal(s) can then be evaporated
and deposited on the substrate to form the doped cesium barium
halide scintillator film. Alternatively, the source material (e.g.,
CsI and BaI.sub.2) can be co-evaporated in separate boats along
with the dopant material (e.g., EuI.sub.2) and subsequently made to
react on a heated substrate to form the monophase of
CsBa.sub.2I.sub.5:Eu. Temperatures for initiating the reaction can
range, e.g., from about room temperature (25.degree. C.) to about
450.degree. C.
As set forth above, scintillator compositions of the present
invention may find use in a wide variety of applications. In one
embodiment, for example, the invention is directed to a method for
detecting energetic (e.g., ionizing) radiation (e.g., gamma-rays,
X-rays, neutron emissions, alpha particles, beta particles and the
like) with high energy resolution using a detector based on a
scintillator described herein. In certain embodiments, the
microcolumnar form of scintillators described herein can be used
for high spatial resolution imaging.
FIG. 5 is a diagram of a detector assembly or radiation detector of
the present invention. The detector 50 includes a doped cesium
barium halide scintillator 52 operably coupled to a light
photodetector 54 or imaging device. The detector assembly 50 can
include a data analysis, or computer, system 56 to process
information from the scintillator 52 and light photodetector 54. In
use, the detector 50 detects energetic radiation emitted form a
source 58.
A data analysis, or computer, system thereof can include, for
example, a module or system to process information (e.g., radiation
detection information) from the detector/photodetectors in an
invention assembly and can include, for example, a wide variety of
proprietary or commercially available computers, electronics, or
systems having one or more processing structures, a personal
computer, mainframe, or the like, with such systems often
comprising data processing hardware and/or software configured to
implement any one (or combination of) the method steps described
herein. Any software will typically comprise machine readable code
of programming instructions embodied in tangible media such as a
memory, a digital or optical recording medium, optical, electrical,
or wireless telemetry signals, or the like, and one or more of
these structures may also be used to transmit data and information
between components of the system in any of a wide variety of
distributed or centralized signal processing architectures.
The detector assembly typically includes material formed from the
scintillator compositions described herein (e.g., SrI.sub.2:Eu,
CaI.sub.2:Eu, CeBr.sub.3, or CsBa.sub.2I.sub.5:Eu scintillators).
The detector further can include, for example, a light detection
assembly including one or more photodetectors. Non-limiting
examples of photodetectors include photomultiplier tubes (PMT),
photodiodes, PIN detectors, charge coupled device (CCD) sensors,
image intensifiers, avalanche detectors and the like. Choice of a
particular photodetector will depend in part on the type of
radiation detector being fabricated and on its intended use of the
device. In certain embodiments, the photodetector may be
position-sensitive. Detectors can further include imaging devices
that can acquire images at high frame rates, such as frame rates
that are faster than about 30 frames per second, about 100 frames
per second, or about 1000 frames per second.
The detector assemblies themselves, which can include the
scintillator and the photodetector assembly, can be connected to a
variety of tools and devices, as mentioned previously. Non-limiting
examples include nuclear weapons monitoring and detection devices,
well-logging tools, and imaging devices, such as nuclear medicine
devices (e.g., PET). Various technologies for operably coupling or
integrating a radiation detector assembly containing a scintillator
to a detection device can be utilized in the present invention,
including various known techniques. In certain embodiments, the
radiation detector comprises a scintillator described herein formed
on a substrate that is optically coupled to the photodetector.
Similarly, scintillator screens including a scintillator described
herein can be included in a radiation detector such that the screen
is optically coupled to the photodetector.
The detectors may also be connected to a visualization interface,
imaging equipment, or digital imaging equipment (e.g., pixilated
flat panel devices). In some embodiments, the scintillator may
serve as a component of a screen scintillator. Energetic radiation,
e.g., X-rays, gamma-rays, neutron, originating from a source, would
interact with the scintillator and be converted into light photons,
which are visualized in the developed film. The film can be
replaced by amorphous silicon position-sensitive photodetectors or
other position-sensitive detectors, such as avalanche diodes and
the like. In some embodiments, neutrons can be indirectly detected
by coupling (e.g., incorporating into or contacting) an absorbing
converter material, such as but not limited to lithium, boron or
gadolinium, into/with a scintillator described herein, and then
detecting emissions (e.g., X-rays and/or alpha particles) produced
by interactions between the neutrons and the absorbing converter
material.
The methods of the present invention further include methods of
performing radiation detection. The methods of performing radiation
detection can include providing a detection device comprising a
scintillator composition including a scintillator described herein;
and a photodetector assembly operably (e.g., optically) coupled to
the scintillator composition; and positioning the device such that
a radiation source is within a field of view of the scintillator
composition so as to detect emissions from the source. Emissions
from the source can include x-rays, gamma-rays, neutrons, alpha
particles, beta particles, or a combination thereof. In certain
embodiments, a material (e.g., a patient, plant, animal, object,
liquid, or gas) can be positioned between the radiation source and
the scintillator composition. In some embodiments, the radiation
source includes a material (e.g., a patient, plant, animal, object,
liquid, or gas). In another embodiment, a material of interest
(e.g., a patient, plant, animal, object, liquid, or gas) may
scatter energetic radiation to the scintillator. The methods of
radiation detection may also include X-ray and gamma ray astronomy
and cosmic ray detection (e.g., in salt mines). In certain
embodiments, a material to be analyzed can be positioned between
the radiation source and the scintillator. In some embodiments, the
radiation source includes a patient. In some embodiments, the
detector can be positioned such that the radiation source is in the
field of view of the scintillator. Alternatively, the radiation
source can be positioned in the field of view of the scintillator
contained in the detector. Also, both the radiation source and the
detector can be moved at the same time such that the radiation
source is in the field of view of the scintillator.
Imaging devices, including medical imaging equipment, such as PET
and SPECT (single-photon emission computed tomography) devices, and
the like, represent other potential applications for the invention
scintillator compositions and radiation detectors. Furthermore,
geological exploration devices, such as well-logging devices, were
mentioned previously and represent an important application for
these radiation detectors. The assembly containing the scintillator
usually includes, for example, an optical window at one end of the
enclosure/casing. The window permits radiation-induced
scintillation light to pass out of the scintillator assembly for
measurement by the photon detection assembly or light-sensing
device (e.g., photomultiplier tube, etc.), which is coupled to the
scintillator assembly. The light-sensing device converts the light
photons emitted from the scintillator into electrical pulses that
may be shaped and digitized, for example, by the associated
electronics. By this general process, gamma rays can be detected,
which in turn provides an analysis of geological formations, such
as rock strata surrounding the drilling bore holes.
In applications of a scintillator composition, including those set
forth above (e.g., nuclear weapons monitoring and detection,
imaging, and well-logging and PET technologies), certain
characteristics of the scintillator are desirable, including its
light output (higher is can be preferred), rise time (faster can be
preferred) and decay time (shorter can be preferred), timing shape
(e.g., fixed or varying, depending upon dopant concentration used
to analyze scintillation events), energy resolution (finer/lower %
can be preferred), spatial resolution (finer, e.g., higher, can be
preferred), and suitable physical properties. The present invention
is expected to provide scintillator materials which can provide the
desired high light output and initial photon intensity
characteristics for demanding applications of the technologies.
Furthermore, the scintillator materials are also expected to be
produced efficiently and economically, and also expected to be
employed in a variety of other devices which require
radiation/signal detection (e.g., gamma-ray, X-ray, neutron
emissions, and the like).
The following examples are provided to illustrate but not limit the
invention.
EXAMPLES
Example 1
Fabrication of SrI.sub.2:Eu (Europium-Doped Strontium Iodide)
Scintillator Films
This example describes fabrication of polycrystalline microcolumnar
scintillator films of europium-doped strontium iodide
(SrI.sub.2:Eu). A wide range of uniform thickness SrI.sub.2:Eu
films can be made ranging from less than 1 mm to greater than 1 cm.
Films having diameters of 7 cm or more can also be fabricated. The
hot wall evaporation methods described herein can be used to make
the SrI.sub.2:Eu films, and since SrI.sub.2:Eu is highly
deliquescent the apparatuses and methods can provide encapsulation
techniques for protecting the materials. Scintillation properties
can be optimized for a variety of applications.
A variety of hot wall evaporation deposition conditions can be
used. FIG. 3 shows an example hot wall evaporation apparatus used
to deposit the SrI.sub.2:Eu films. In this example, 20 grams of
SrI.sub.2 was placed in one precursor boat and 2.5 grams of
EuI.sub.2 was placed in a second precursor boat. The maximum
evaporation temperature was about 800.degree. C. The base pressure
in the apparatus was 10.sup.-5 ton. Deposition time was three hours
with 30 minutes of dwell time at 800.degree. C. and 2.5 hours of
ramp-up and ramp-down time total. The thickness of the film was 7
mm maximum with a slight parabolic profile. Another film was
fabricated that was 38 mm in diameter and approximately 7 mm thick.
The light emission of the film was 59,500 ph/MeV, as normalized to
the quantum efficiency of the measuring system. SrI.sub.2:Eu had an
emission wavelength at 435 nm. The films were encapsulated,
hermetically sealed in an aluminum enclosure with a quartz window
on one side. Residual window sealant in the capsule reacted with
the SrI.sub.2:Eu causing white and yellow discoloration around the
periphery. As shown in FIG. 6, scanning electron microscopy showed
a microcolumnar structure of the deposited SrI.sub.2:Eu film, which
can improve imaging resolution by channeling light to the surface
of the film and to a photodetector.
Gamma ray spectra were compared between a 1.2 mm thin film of
SrI.sub.2:Eu and a single crystal of LaBr.sub.3:Ce. The shaping
time for LaBr.sub.3:Ce was 0.25 microseconds, and the shaping time
for SrI.sub.2:Eu was 4 microseconds. The gamma source was Co-57
(122 keV). The gamma light yield for the LaBr.sub.3:Ce single
crystal was 52,000 ph/MeV and 59,500 ph/MeV for the 1.2 mm thin
film of SrI.sub.2:Eu.
Example 2
Fabrication of CeBr.sub.3 Scintillator Films
This example describes growth of CeBr.sub.3 films using hot wall
evaporation, the method that can produce relatively inexpensive
large area, thick films in desired shapes or sizes. The CeBr.sub.3
material could be grown in polycrystalline form suitable for
spectroscopic applications, or even in the microcolumnar form
desirable for high spatial resolution gamma ray imaging. Films
produced using this approach possess all of the excellent
properties of their melt-grown crystal counterparts. FIG. 7A shows
light yield and energy resolution of as-fabricated 1 mm CeBr.sub.3
film approaching that of a LaBr.sub.3:Ce commercial single crystal.
FIG. 7B is a response of a 20 micron thick CeBr.sub.3 film to 5.5
MeV alpha particles from .sup.241Am source demonstrating high
detection efficiency and excellent gamma ray rejection. Also
plotted are the response of the same film to 60 keV .sup.241Am, 662
keV .sup.137Cs, and 122 keV .sup.57Co gamma rays, showing excellent
gamma ray discrimination.
Example 3
Pre-Synthesis of Ba.sub.2CsI.sub.5:Eu Polycrystals for Fabrication
of CsBa.sub.2I.sub.5:Eu Scintillator Films
Vertical Bridgman Process:
This example describes pre-synthesis of polycrystals of
europium-doped, ternary inorganic compound Ba.sub.2CsI.sub.5.
Pre-synthesis Ba.sub.2CsI.sub.5:Eu polycrystals used a simplified
vertical Bridgman's method. The polycrystals were grown from CsI,
BaI.sub.2 and EuI.sub.2 powder beads. The quartz ampoules
containing this admixture were evacuated, sealed and then heated in
the furnace up to 700 C, beyond its melting point. This provided
for the formation of a homogenous liquid melt which was conducive
for the formation of Ba.sub.2CsI.sub.5:Eu singular- or mono-phase.
Several such growth runs were performed to synthesize
"Ba.sub.2CsI.sub.5:Eu pre-synthesis charges" with varying
concentrations of Eu.sup.2+ in the range of 1 to 10% (mole %), to
study the effect of Eu dopant concentration on scintillation
properties.
Light Yield (Pre-Synthesis):
As the Ba.sub.2CsI.sub.5:Eu scintillators in their polycrystalline
formats with variations in Eu concentrations were fabricated, we
performed detailed characterizations of their light yield. The
optical signal produced by a specimen when irradiated with a
.sup.57Co source (122 keV photons) were measured using a PMT
coupled to standard NIM electronics (Can berra #2003 preamplifier,
Can berra #2020 amplifier), and the spectrum were recorded using an
Oxford Instruments MCA. A commercial single-crystal LaBr.sub.3:Ce
scintillator with known light yield of 55,000 photons/MeV available
at RMD was used as a standard for comparison. By comparing the 122
keV photopeaks in all cases, the relative light output of various
Ba.sub.2CsI.sub.5:Eu specimens was determined. The light output of
Ba.sub.2CsI.sub.5:Eu polycrystals varied as Eu concentrations
changed. The highest light output was estimated to be .about.70,000
ph/Mev at 5% Eu by mole %, taking into account the ratio between
the centroid positions and quantum efficiencies of the super
bi-alkali PMT for the LaBr.sub.3:Ce reference and
Ba.sub.2CsI.sub.5:Eu crystals. This result gave insight into the
stoichiometry in Ba.sub.2CsI.sub.5:Eu films for achieving a bright
material.
Afterglow (Pre-Synthesis):
Due to the high speed nature of CT imaging, the afterglow of the
scintillator is a important parameter for CT imaging. To minimize
artifacts arising from excessive persistence, afterglow of a
scintillator should be minimal. Therefore, the afterglow
measurements of Ba.sub.2CsI.sub.5:Eu are useful in ascertaining its
use for CT applications. These measurements were performed by
exciting the polycrystal specimens by means of a Golden Engineering
XRS-3 source, which provides X-ray pulses 20 ns FWHM, with nominal
maximum photon energy of 250 kVp. The scintillation response from
the specimens is passed through a 0.2-m McPherson monochromator,
detected by a Hamamatsu R2059 photomultiplier, and re-corded by a
Tektronix TDS220 digital storage oscilloscope. The pre-synthesized
polycrystals demonstrated low afterglow, which is important for CT
applications. For example, the polycrystals showed the afterglow
results after 2 ms interval following an X-ray pulse, for different
specimens as a function of its Eu dopant concentration. The trend
seems to show that as the Eu concentration increases, the afterglow
increases as well. The afterglow of 0.3% for Ba.sub.2CsI.sub.5:5%
Eu is notable considering the current standard CsI:Tl whose
afterglow is nearly 5-10% at 2 ms.
X-Ray Diffraction Studies:
One aspect of the pre-synthesis polycrystals is the formation of
Ba.sub.2CsI.sub.5:Eu "mono-phase". Phase identification was
performed on the pre-synthesis polycrystals of Ba.sub.2CsI.sub.5:Eu
by powder X-ray diffraction (XRD). The measurement was performed
with a Bruker Nonius FR591 rotating anode X-ray generator equipped
with a Cu target at a 50 kV and 60 mA electron beam.
XRD patterns for three polycrystal samples were obtained as well as
the standard diffraction pattern of Ba.sub.2CsI.sub.5 calculated
from single-crystal data. PC100A is as-fabricated sample, while
PC100B-V specimen was subjected to high-vacuum (10.sup.-6 Ton) for
24 hours post-fabrication. The sample PC100A-VH sample was annealed
at 100-125.degree. C. in high-vacuum for 1 hour. The peak positions
of all three samples match well with the pattern derived from the
single crystal structure. No impurities corresponding to reactants
or undesired products were visible, thus confirming that the 3
samples are Ba.sub.2CsI.sub.5. However, the degree of crystallinity
differed from sample to sample, and as expected, annealed sample
(PC100A-VH) exhibited better crystallinity than the rest. While not
being bound by any particular theory, this can be explained by the
presence of free iodine in the crystal matrix of Ba.sub.2CsI.sub.5.
The source of free iodine is the anion part of EuI.sub.2, which is
added during the pre-synthesis fabrication by Bridgman's process.
It is also possible for factory-shipped BaI.sub.2 to have excess
iodine. In any case, the excess iodine precipitates on the grain
boundaries and hinders crystallization of Ba.sub.2CsI.sub.5 during
the crystal growth. Hence as a result, it can be seen that
post-fabrication annealing helps de-gas the excess iodine in vacuum
and improves its crystallinity for producing highly crystalline
mono-phase Ba.sub.2CsI.sub.5:Eu. From the standard reference it is
deduced that single-crystal Ba.sub.2CsI.sub.5:Eu has a monoclinic
P2.sub.1/c space group with cell parameters a=10.541 .ANG., b=9.256
.ANG. c=14.637 .ANG.; .beta.=90.194.degree..
Compositional Analysis (Polycrystals):
The preliminary qualitative compositional analysis was carried out
using X-ray fluorescence (XRF) microscopy. It is a non-destructive
technique that utilizes the photoelectric interaction of radiation
with matter to produce a signal that is proportional to elemental
concentration. The basic principle is that ionizing radiation
induces photoelectric absorption in the test material for the
elements of interest, followed by emission of characteristic X-rays
that can be detected, isolated and counted. We have adapted a
portable device, LeadTracer-ROHS.TM. from RMD Instruments Inc.
originally designed for the screening of lead and mercury, to
quantify the Cs, Ba, I and Eu concentrations in
Ba.sub.2CsI.sub.5:Eu scintillator. It uses a .sup.57Co source to
excite the sample with 122 keV gamma rays, and then detects the
resulting X-ray fluorescence from the sample using a CdTe detector
that provides .about.700 eV energy resolution. The compositional
analysis showed the formation of the "mono-phase" of
Ba.sub.2CsI.sub.5 in the pre-synthesis step.
Example 3
Fabrication of CsBa.sub.2I.sub.5:Eu Scintillator Films with
Physical Vapor Deposition
Ba.sub.2CsI.sub.5:Eu is a scintillator, well suited for
next-generation radiography. Ba.sub.2CsI.sub.5:Eu has an estimated
light yield of .about.100,000 ph/MeV, and its X-ray excited
emission wavelength, detected around 430 nm, can be tailored to
match the response of CMOS, CCDs or a-Si:H flat panel detectors.
The higher density of Ba.sub.2CsI.sub.5:Eu (5.04 gm/cm.sup.3)
affords higher stopping power than CsI:Tl (4.53 gm/cm.sup.3).
As-deposited films of Ba.sub.2CsI.sub.5:Eu, fabricated via physical
vapor deposition, exhibit highly structured morphology, which can
be optimized either for high spatial resolution or high light
output. Its fast decay and low afterglow make Ba.sub.2CsI.sub.5:Eu
ideally suited for high frame-rate applications.
The next generation of medical, security, industrial and scientific
X-ray imaging systems demands a high performance scintillator with
outstanding properties of high sensitivity, high resolution, high
brightness, high speed, and favorable energy of light emission.
Here, this example reports on Ba.sub.2CsI.sub.5:Eu, a novel
high-performance scintillator well suited for such radiography,
fabricated as a thin film via physical vapor deposition (PVD) onto
a suitable (typically radiolucent) substrate. Ba.sub.2CsI.sub.5:Eu
is a bright scintillator with an estimated light yield of
.about.100,000 photons/MeV. While the peak X-ray excited emission
wavelength of both crystalline and thin film Ba.sub.2CsI.sub.5:Eu
are 430 nm, the emission spectrum of the thin film form shows
significant broadening compared to the crystal sample. This implies
that the emission properties can be tailored to match the
sensitivities of photodetectors such as CMOS, CCDs or a-Si:H flat
panels. The density of Ba.sub.2CsI.sub.5:Eu (5.04 gm/cm.sup.3) is
higher than traditional sensors such as CsI:Tl (4.5 gm/cm.sup.3),
and thus affords higher stopping power. The PVD-deposited films
exhibit structured cross-sections, which can be optimized either
for high spatial resolution in amorphous microcolumnar structure
(AMS) form or for high light output in crystalline microcolumnar
structure (CMS) form. The decay characteristics of
Ba.sub.2CsI.sub.5:Eu demonstrate <0.05% afterglow at 2 ms
following X-ray excitation, making it ideally suited for high frame
rate applications such as fluoroscopy and cone beam CT (CBCT).
This example describes a scintillator, Ba.sub.2CsI.sub.5:Eu, (FIG.
8A) as an X-ray sensor material for next-generation radiography,
and demonstrates its efficacy for high-sensitivity, high-resolution
X-ray imaging through its integration into existing imaging
detectors. This recently discovered scintillator is fast and offers
effectively unprecedented scintillation efficiency of
.about.100,000 optical ph/MeV. Its emission, centered around
.about.430 nm (FIG. 8B), can be tailored to suit CMOS, CCDs or
a-Si:H flat panel detectors. This bright emission, along with its
afterglow-free fast decay, makes it ideally suited for high frame
rate imaging and other "light-starved" applications.
Ba.sub.2CsI.sub.5:Eu is fabricated via the physical vapor
deposition (PVD) method of thermal evaporation in microcolumnar
form, a structure that minimizes the traditional tradeoff between
spatial resolution and detector X-ray absorption efficiency. Films
fabricated in crystalline microcolumnar structure form can be used
to retrofit pixelated scintillators in existing CT systems.
Amorphous microcolumnar structure forms can be directly deposited
onto flat panel detector arrays suitable for new dedicated,
organ-specific cone beam CT (CBCT) or radiography systems. Both
forms can be grown in dimensions suitable for conventional
photodiode coupling or for coupling to large-format CMOS, CCDs or
a-Si:H flat panel detectors, and their thickness can be tailored to
maximize absorption efficiency for 140 kVp X-rays typically used in
medical CT.
Ba.sub.2CsI.sub.5:Eu is a new material and is not yet available
commercially in a form amenable to our vapor deposition growth
needs. Consequently, in order to pursue PVD fabrication, melt-grown
crystals were pre-synthesized at using a vertical Bridgman furnace.
Several growth runs were performed to synthesize these crystals,
with varying concentrations of Eu.sup.2+ dopant. FIG. 9 shows some
of these crystals under normal and UV illumination. Subsequently,
the pre-synthesized crystals were thermally evaporated in a hot
wall crucible under high vacuum conditions onto a quartz substrate.
The films fabricated so far are only .about.30 .mu.m thick, but
they are 5 times brighter than conventional MinR 2000 screens under
identical exposure conditions. More light is anticipated with
thicker films, due to higher X-ray absorption. The X-ray excited
emission spectrum of the film shows significant broadening compared
to that of a crystal (see FIG. 8B). These data demonstrate that
emission properties may be tailored to match the response of the
photodetector.
This example describes a vapor-deposited ternary compound in a
microcolumnar format. This material, in the described CMS and AMS
morphologies, is expected to be a breakthrough for the radiography
community in general and medical imaging in particular.
Additionally, owing to its fast decay and low afterglow,
Ba.sub.2CsI.sub.5:Eu is well suited for high speed radiography,
fluoroscopy or CBCT applications. This material has the potential
to provide high spatial resolution along with high brightness and
enhanced X-ray absorption, enabling development of detectors with
high detective quantum efficiency (DQE(f)).
This example focuses on the fabrication and characterization of a
novel scintillator, Ba.sub.2CsI.sub.5:Eu, in microcolumnar forms
that possess traits to qualify as the next generation sensor for
radiography. Practical and cost-effective designs and processes
have been identified to fabricate Ba.sub.2CsI.sub.5:Eu films
suitable for use in X-ray imaging and to integrate them into
current detectors, which make it a promising, high-performance
scintillator for demanding applications.
Example 4
Fabrication of CsBa.sub.2I.sub.5:Eu Scintillator Films with Hot
Wall Evaporation and Thermal Evaporation
This portion of the example describes a Ba.sub.2CsI.sub.5:Eu
scintillator in a crystalline microcolumnar (CMS) and amorphous
microcolumnar (AMS) format, made using hot wall evaporation. An HWE
apparatus similar to the apparatus in FIG. 4 can be used to make
the Ba.sub.2CsI.sub.5:Eu films. Numerous CMS samples were
fabricated via HWE method using the pre-synthesis polycrystals with
variation in dopant concentration from 1 to 15%. Prior to any
characterization, the films were protected from moisture by
performing a barrier coating of 5 to 8 .mu.m film of Parylene C.
The coated samples were further protected by housing it in an
aluminum casing, whose hermetic sealing was carried out in a
glovebox with <0.5 ppm moisture and oxygen ambience. One example
scintillator was a hermetically sealed Ba.sub.2CsI.sub.5:Eu sample
measuring 100 .mu.m in thickness and 6 cm in diameter (on a quartz
substrate).
Light Yield:
The light yield for CMS sample was measured using the super
bi-alkali PMT setup as described previously for the polycrystalline
samples. The light yield was estimated in reference to the
commercial LaBr.sub.3:Ce standard and by accounting for the
detector quantum efficiency for its emission wavelength (380 nm)
with respect to Ba.sub.2CsI.sub.5:Eu(450 nm). FIG. 10 shows the
light output of the films as a function of its nominal Eu
concentration in its starting material (pre-synthesized material).
The trend in light yield for CMS films and their starting materials
trace each other very closely, as the Eu doping concentration is
varied from 1 to 15%. This examples demonstrates the capability of
an optimized HWE process to reproduce melt-grown crystal's
scintillation properties in CMS films. The best performance in
terms of brightness was measured in CMS film with 5% Eu and is
estimated be me >70,000 ph/MeV.
This portion of the example describes fabrication of microcolumnar
Ba.sub.2CsI.sub.5:Eu scintillator films using thermal evaporation.
The resulting microcolumnar structure conserves and promotes
channeling of the scintillation light by means of total internal
reflection, thereby suppressing lateral light spread within the
film. This allows, e.g., fabrication of a thicker film for higher
X-ray absorption without sacrificing spatial resolution for cone
beam CT (CBCT) applications.
Thermal Evaporation:
The schematic for the thermal evaporation used for fabrication of
AMS Ba.sub.2CsI.sub.5:Eu films is shown FIG. 11. The deposition was
carried out by evaporating a pre-calculated admixture of BaI.sub.2,
CsI and EuI.sub.2 beads from a single heated crucible. The
substrates were loaded on the planetary tool plates, which
permitted angular deposition on their surfaces and the ambient
substrate temperatures were maintained at 50-60.degree. C., all of
which favored AMS growth. The admixture was heated to 350.degree.
C. in the crucible under vacuum condition for 2 hours to dehydrate
and following which it was heated to 600.degree. C. for the
formation of liquid melt phase of Ba.sub.2CsI.sub.5:Eu. The
evaporation was then proceeded by opening the shutter between the
charge and the rotating substrates. Aluminum coated (reflector)
graphite substrates (low-Z) were used as substrates in this
endeavor. Ba.sub.2CsI.sub.5:Eu material is somewhat hygroscopic in
nature and its water uptake from atmospheric moisture can be
detrimental to the scintillator performance. Hence it can be
important to protect the scintillator plated by providing a good
moisture barrier coating. Parylene C coats on the as-deposited film
(e.g., 5-7 .mu.m) provided a good moisture barrier protection
without altering the light transmission properties.
Microcolumnar Morphology:
Scanning electron microscopy (SEM) was performed on the
cross-section of a freshly cleaved AMS Ba.sub.2CsI.sub.5:Eu film.
FIG. 12 shows the SEM characterization of the microcolumnar
morphology of the film along with the Parylene protection on the
top surface. The film thickness measures a total of .about.50 .mu.m
and the column diameters are .about.0.5 to 1 .mu.m.
Spatial Resolution Measurements:
The spatial resolution of the scintillators were evaluated by
measuring a pre-sampled line spread function (LSF) followed by a
fast Fourier transform (FFT) of the LSF to obtain the modulation
transfer function (MTF(f)). A 10 gm wide tantalum slit oriented at
a <1.degree. angle relative to the CCD pixel row (or column)
direction was imaged to obtain the pre-sampled LSF with a sampling
interval of 0.7 .mu.m or less. The acquired MTF data showed that a
50 .mu.m thick Ba.sub.2CsI.sub.5:Eu AMS film exhibited resolution
of 6 LP/mm (.about.80 .mu.m) spatial resolution as a function of
spatial frequency.
Emission Spectrum:
The X-ray excited emission spectra of various scintillator samples
was measured using an existing setup, with the sample under
investigation excited by the 8 keV Cu K.alpha. line. To generate
the required flux at the sample, the X-ray generator was operated
at 40 kV with 20 mA current. The resulting scintillation light was
collected in a MacPherson 0.2 m monochromator (model 234/302) that
separates the light into its wavelength components. The intensity
of the selected wavelength was registered using an RCA model C31034
photomultiplier tube (PMT). The operation of the whole instrument,
including the X-ray trigger, the rotation of the monochromator to
select the wavelengths, and the data acquisition and analysis was
software controlled.
As shown in FIG. 13, the as-deposited Ba.sub.2CsI.sub.5:Eu AMS
sample emitted at .about.450 nm which is characteristic of
Eu.sup.2+ dopant activity. When the sample was annealed at 160 C
for 40 hours, the emission spectra showed a significant number of
photons (60%) had red-shifted to 550 nm, in addition to its main
emission at 450 nm. This is can be useful for CBCT as most of the
sensors (CCDs, CMOS, or a-Si:H flat panels) have high sensitivity
in 550 nm (green) range. While, not bound by any particular theory,
this shift may be due to the rearrangement within the lattice,
where the Eu.sup.2+ impurity moves from interstitial to lattice
positions, allowing transitions that favor green emission.
X-Ray Characterization:
Light output measurements were made by coupling AMS
Ba.sub.2CsI.sub.5:Eu films measuring 50 .mu.m thickness (20
mg/cm.sup.2) to an electron multiplying CCD via a 3:1 fiber optic
taper. The film was exposed to a uniform flood field of 70 kVp
X-rays and resulting analog-to-digital unit values (ADUs) were
averaged over a pre-defined region of interest. Similar measurement
was repeated on commercial Gd.sub.2O.sub.2S (GOS) screen
(MinR-2000, 34 mg/cm.sup.2) for the sake of comparison. The
as-deposited AMS film measured 450 ADUs compared to 1800 ADUs for
MinR under similar X-ray conditions. This is primarily due to very
low (<5%) quantum efficiency of the CCD to 450 nm wavelength of
Ba.sub.2CsI.sub.5:Eu compared to 55% QE for the GOS 540 nm light.
By correcting for quantum efficiencies of the CCD at different
emissions and by accounting for X-ray absorptions arising from
different mass thicknesses of the two scintillators, the estimated
brightness for Ba.sub.2CsI.sub.5:Eu film is 13,500 ADUs compared to
MinR's 3400 ADUs. This result implies that Ba.sub.2CsI.sub.5:Eu
scintillator's light output is almost 4 times greater than
commercially available GOS screens. The as-deposited samples were
annealed in Ar environment for 40 hours at 160 C Annealing process
remove defects in the films, remove any non-uniformity in dopant
distribution and improve the microcolumnar morphology. And as a
result, a 11% increase in the ADU was registered following
annealing of the film.
Dual Doping with Tl+Eu:
Using the schematic for the thermal evaporation described above,
fabrication of dual doped AMS Ba.sub.2CsI.sub.5 films were
produced. Dual doping can induce a red shift emission spectrum for
the Ba.sub.2CsI.sub.5 host from 450 to 550 nm, to, e.g., improve
its sensitivity to CCD, a-Si:H, or SiPM detectors. The dopants
selected to achieve this effect were thallium and europium. The
deposition was carried out by evaporating polycrystals of
Ba.sub.2CsI.sub.5:5% Eu and thallium iodide (TlI) beads in two
independently controlled tantalum crucibles. The substrates were
loaded on the planetary tool plates, which permitted angular
deposition on their surfaces and the ambient substrate temperatures
were maintained at 50-60.degree. C., all of which favored AMS
growth. The resulting films were 60 .mu.m thick and exhibited
microcolumnar morphology and line-pair resolution. The spectral
emission of dual-doped Ba.sub.2CsI.sub.5:Eu,Tl films is shown in
FIG. 14.
Light output measurements were made by coupling AMS
Ba.sub.2CsI.sub.5:Eu,Tl films measuring 60 .mu.m thickness (24
mg/cm.sup.2) to an electron multiplying CCD via a 3:1 fiber optic
taper. The film was exposed to a uniform flood field of 70 kVp
X-rays, in a setup as described previously. The as-deposited AMS
film measured 2880 ADUs compared to 1800 ADUs for MinR 2000
standard GOS screen under similar X-ray conditions. Dual doped film
and MinR 2000 practically have the same emission wavelength and
hence do not need any correction for quantum efficiencies. However,
by correcting for X-ray absorptions arising from different mass
thicknesses of the two scintillators, the estimated brightness for
dual doped Ba.sub.2CsI.sub.5:Eu,Tl film is 2 times greater than
commercially available GOS screens. A more orchestrated approach is
needed to understand the collective effect of multiple activators
in any given scintillator host, nevertheless dual doping of
Ba.sub.2CsI.sub.5 is a promising pathway to engineer its emission
wavelength for improving its sensitivity to various detectors such
as CCD, a-Si:H, and SiPMs etc.
Example 5
Imaging with CsBa.sub.2I.sub.5:Eu Scintillator Films
X-Ray Radiographic Imaging:
To illustrate the X-ray imaging capability, Ba.sub.2CsI.sub.5:Eu
films were coupled to an EMCCD via a 3:1 taper. The X-ray
source-to-detector distance was 45 cm, and the imaged object was
placed in contact with the CCD front end. Thus, there is no optical
magnification in registered images and resolution reflects true
resolution of the film (EMCCD has a very high intrinsic resolution
of 39 .mu.m, or .about.13 lp/mm). FIG. 15 shows an acquired
radiography image of a line-pair phantom. The image shows excellent
spatial resolution, especially the line-pair phantom which
demonstrates 6 LP/mm resolution and contrast (CTF(f)) consistent
with modulated transfer function (MTF(f)) data.
Dynamic X-Ray Imaging:
Dynamic X-ray imaging with a Ba.sub.2CsI.sub.5:Eu film was
evaluated. To this end, dynamic images of a moving toy were
acquired by our custom developed CMOS X-ray camera capable of
providing 1 k.times.1 k pixel images at 2000 frames per second.
With a reduced field of view, this detector can acquire images at
up to 120,000 fps. As such this is an ideal tool to evaluate
scintillator screens for high frame rate imaging such as the
current CBCT application. During this period, the RMD AMS
Ba.sub.2CsI.sub.5:Eu film was pressure coupled to this detector and
images were acquired at 1000 fps using a Hamamatsu microfocus X-ray
source operating at 100 kVp and 100 .mu.A. A single frame of a
sequence of images was acquired at 1000 fps. As is evident from the
image, the camera (film) shows excellent dynamic range as the
plastic gears are clearly visible against the high contrast
stainless steel components. Even the plastic gear system inside the
rectangular metal frame is clearly visible.
It is understood that the examples and embodiments described herein
are for illustrative purposes only and that various modifications
or changes in light thereof may be suggested to persons skilled in
the art and are included within the spirit and purview of this
application and scope of the appended claims. Numerous different
combinations of embodiments described herein are possible, and such
combinations are considered part of the present invention. In
addition, all features discussed in connection with any one
embodiment herein can be readily adapted for use in other
embodiments herein. The use of different terms or reference
numerals for similar features in different embodiments does not
necessarily imply differences other than those which may be
expressly set forth. Accordingly, the present invention is intended
to be described solely by reference to the appended claims, and not
limited to the preferred embodiments disclosed herein.
* * * * *